13.3 Raman and Brillouin scattering

Raman and Brillouin scattering are inelastic light scattering processes that reveal material properties. Raman scattering interacts with molecular vibrations, while Brillouin scattering involves acoustic waves. Both provide valuable insights into material structure and composition.

These techniques offer non-destructive ways to study materials in various states and environments. Raman spectroscopy identifies chemical compounds and analyzes crystal structures, while Brillouin spectroscopy determines elastic constants and studies phase transitions. They're widely used in material characterization and sensing applications.

Raman and Brillouin Scattering

Principles of Raman and Brillouin scattering

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• Raman and Brillouin scattering are inelastic scattering processes where incident photons interact with the material, exchanging energy and momentum, resulting in scattered photons having a different frequency than the incident photons (Stokes and anti-Stokes scattering)
• Raman scattering involves the interaction of photons with molecular vibrations or phonons
  • Stokes scattering occurs when a photon loses energy and the molecule transitions to a higher vibrational state (red-shifted)
  • Anti-Stokes scattering occurs when a photon gains energy and the molecule transitions to a lower vibrational state (blue-shifted)
• Brillouin scattering involves the interaction of photons with acoustic waves or phonons, which are collective oscillations of atoms or molecules in a material (sound waves)
  • Scattered photons gain or lose energy equal to the energy of the acoustic phonon, resulting in a frequency shift proportional to the velocity of sound in the material

Raman vs Brillouin scattering characteristics

• Origin of the scattering
  • Raman scattering originates from the interaction with molecular vibrations or optical phonons (intramolecular bonds)
  • Brillouin scattering originates from the interaction with acoustic waves or acoustic phonons (intermolecular interactions)
• Frequency shift
  • Raman scattering typically exhibits frequency shifts in the range of $10^{12}$ to $10^{14}$ Hz (THz range)
  • Brillouin scattering typically exhibits frequency shifts in the range of $10^{9}$ to $10^{11}$ Hz (GHz range)
• Spectral linewidth
  • Raman scattering produces relatively broad spectral lines due to the short lifetime of molecular vibrations ($10^{-12}$ to $10^{-14}$ s)
  • Brillouin scattering produces narrow spectral lines due to the longer lifetime of acoustic phonons ($10^{-9}$ to $10^{-11}$ s)
• Scattering geometry
  • Raman scattering can be observed in any direction relative to the incident light (isotropic)
  • Brillouin scattering is strongly dependent on the scattering angle, with maximum intensity observed at specific angles determined by the acoustic wave velocity and the refractive index of the material (anisotropic)

Material properties from scattering analysis

• Raman scattering provides information about the molecular structure and composition of a material
  • Raman spectra contain peaks corresponding to specific molecular vibrations (fingerprint region)
  • Peak positions, intensities, and shapes can be used to identify molecules and their environments (crystallinity, stress, temperature)
  • Enables the study of chemical bonding, phase transitions, and molecular interactions
• Brillouin scattering provides information about the elastic and acoustic properties of a material
  • Frequency shift of the scattered light is related to the velocity of acoustic waves in the material, which depends on the elastic constants (Young's modulus, shear modulus) and density of the material
  • Allows the determination of elastic constants, study of phase transitions (solid-liquid), and characterization of thin films and nanostructures (thickness, interfaces)
• Both techniques are non-destructive and can be used to probe the properties of materials in various states (solid, liquid, gas) and environments (high pressure, high temperature)

Applications in characterization and sensing

• Material characterization
  1. Raman spectroscopy: identification of chemical compounds (polymers, pharmaceuticals), analysis of crystal structure and phase (graphene, diamond), and detection of defects and impurities (doping, strain)
  2. Brillouin spectroscopy: determination of elastic constants (ceramics, composites), study of phase transitions (glass transition), and characterization of thin films and nanostructures (semiconductor layers, optical coatings)
• Sensing applications
  1. Raman scattering: detection of specific molecules or chemical species (explosives, drugs), monitoring of chemical reactions (polymerization, catalysis), and analysis of biological samples (cells, tissues)
  2. Brillouin scattering: measurement of temperature and pressure (geophysics, oceanography), monitoring of mechanical stress and strain (structural health monitoring), and detection of flow and turbulence in fluids (aerodynamics, microfluidics)
• Remote sensing and imaging
  • Raman and Brillouin scattering can be used for remote sensing and imaging of materials and structures (standoff detection)
  • Applications include the analysis of artwork and historical artifacts (pigments, binders), monitoring of industrial processes (quality control), and detection of hazardous materials (chemical warfare agents)
  • Enables non-contact and non-invasive measurements with high spatial resolution (confocal microscopy) and depth profiling capabilities (optical sectioning)